1. Field of the Invention
The invention relates to an electric motor, in particular to a supporting arrangement for a bearing in a high speed electric motor.
2. Description of the Related Art
An electrical motor comprises a housing, and a rotating shaft extending through the housing. A stator is provided on the inner surface of the housing. The stator includes a laminated stator core and a system of coils. A rotor is mounted on the rotating shaft. The rotor comprises a laminated rotor core and a conductive portion. The rotating shaft is supported, at either end thereof, in the housing and for rotation, by bearings such as ball bearings.
After the motor starts, the temperature of the motor increases due to the copper loss and the iron loss of the stator and the rotor. The increased temperature results in the thermal expansion of the elements of the motor. On the other hand, after the motor stops, the temperature of the motor decreases. The decreased temperature results in the thermal contraction of the elements. The increasing and decreasing rates of the temperature are not coincident between the stator and the rotor due to, for example, differences of the loss of the stator and the rotor, materials of the stator and the rotor, and the cooling arrangements of the motor.
Thus, a temperature difference appears between the stator and the rotor. The differences in the temperature and the material between the rotor and the stator results in a difference in thermal expansion and contraction of rotor elements, which include the rotor and the shaft, and the stator elements, which include the stator and the housing. The difference in the thermal expansion and contraction takes place, in particular, along the axis of the rotating shaft.
If the bearing cannot move in the axial direction, pressure is applied to the bearings, which can make the bearings deform and seize, due to the thermal expansion and contraction. In order to prevent such damage, in the prior art, a supporting arrangement for the bearings is provided to allow the one of the bearings to move along the axis. The supporting arrangement comprises a support in the form of a ring into which the bearing is fitted. The support is fitted into a bore in the housing. The outer surface of the support and the inner surface of the bore provide sliding surfaces to allow the support to move in the axial direction.
Fretting corrosion, which is well known, is surface damage that occurs between two surfaces of solid bodies which are in close contact, under pressure and subject to a micro-mechanical vibration or slight relative motion, in particular, in the tangential direction. Fretting corrosion occurs, for example, on the surface of a fitted shaft and between contacting surfaces of a gear coupling. The fretting corrosion results in problems such as seizing, and the generation of vibration and noise. Mechanical factors such as the frequency and amplitude of the vibration, the pressure on the surfaces, and the number of cycles, physical factors such as the material and the surface hardness, and chemical factors such as temperature and lubrication affect the development of fretting corrosion.
As described above, the supporting arrangement for the bearings allows one of the bearings to move in the axial direction. Therefore, fretting corrosion can occur between the outer surface of the support and the inner surface of the bore, into which the support is fitted, due to the vibration of the motor itself or an external device connected to the motor. In particular, if a motor has a rated speed higher than 1,000 RPM, fretting corrosion often occurs due to the high frequency of the vibration.
In the prior art, many improvements have been provided to prevent fretting corrosion. The improvements include, for example, reducing the clearance between the sliding surfaces in the supporting arrangement, providing a chrome plating on one or both surfaces to increase the surface hardness, or providing special lubrication between the surfaces. A lubricant of molybdenum grease is well known that it is effective to reduce the fretting corrosion.
The prior art supporting arrangement for the bearing, which uses the molybdenum lubricant, has a problem in that the lubricant can come out of the clearance between the sliding surfaces due to the vibration and thermal expansion and contraction. Thus, the fretting corrosion would occur a relatively short period after filling the clearance with the molybdenum lubricant. The fretting corrosion in the supporting arrangement makes the bearing subject the pressure due to the thermal expansion of the shaft, which results in the deformation and seizing of the bearing.